1. Field of the Invention
This invention relates generally to automotive power steering gears and, more particularly, to variable effort power steering gears where power assist varies with vehicle speed.
2. Description of the Prior Art
In typical rotary valve type automotive power steering gears, a torsion bar is disposed between an input or stub shaft connected to the steering wheel of the vehicle and an output or pinion shaft connected to the steered road wheels of the vehicle. With no manual steering torque at the steering wheel, the torsion bar maintains a balanced, open center relationship between a valve spool on the stub shaft and a valve sleeve rotatable as a unit with the pinion shaft. In addition, the torsion bar resiliently resists angular deflection of the steering wheel and stub shaft relative to the pinion shaft which resistance is sensed by the operator as "road feel". Ideally, road feel is minimum during low speed or parking maneuvers and maximum at higher vehicle speeds normally associated with highway driving. To that end, detent type variable effort power steering gears have been proposed wherein one or more detents are slidably received in radial bores in a housing connected to the pinion shaft and are urged by a servo fluid pressure proportional to vehicle speed into grooves in the stub shaft. The detents connect the stub shaft to the pinion shaft for unitary rotation until the manual torque applied at the steering wheel achieves a threshold magnitude sufficient to ramp the detents out of the grooves. Thereafter, as the stub shaft rotates relative to the pinion shaft, the rotary valve operates in its usual fashion to port pressurized fluid to the steering assist fluid motor. An automotive power steering gear according to this invention is an improved detent type variable effort power steering gear.